The development of a vaccine for HIV/AIDS is an urgent priority. Millions of new HIV infections occur each year, and the rate of these infections is on the increase in developing countries. These areas have no access to antiretroviral therapy, so the only hope is development of a safe and efficacious vaccine. The most useful model for vaccine development has been the SIV-infected macaque. While a number of vaccine strategies have been tested in this model system, results are often complicated by the use of different isolates of SIV, which vary in their pathogenicity. The most efficacious vaccines that have been tested are the gene-deleted live attenuated SIVs (SIV delta nef and its derivatives). However, the effectiveness of these vaccines are complicated by safety issues (disease development in neonates vaccinated with virus), and the delay, following vaccination, to reach the maximum protective effect. DNA-based immunizations have recently been shown to effectively prime immune responses to HIV and SIV antigens. However, DNA alone cannot induce protection and requires a boost, such as a recombinant poxvirus expressing SIV antigens. Still, it is unknown if this methodology will afford long-term protection. Because of the efficacy of live attenuated viruses at generating protective immunity and because of the strong priming induced by DNA-based technologies, we hypothesize that a combination of these two vaccine systems may provide an excellent combined regimen for vaccination against immunodeficiency viruses. To test this hypothesis, we propose to compare the immunogenicity (in macaques) of a DNA immunization followed by a live attenuated boost, with that of a live attenuated virus administration alone. Humoral and cellular immune responses will be fully characterized, as will viral parameters following live attenuated virus boost. Animals will then be challenged with the pathogenic SHIV89.6p following live attenuated virus administration. Challenged animals will be evaluated for protection from infection and for protection from pathogenic effects (CD4+ T cell decline, high viral loads). These studies will allow a comparison between a combination DNA/live attenuated virus immunization strategy and a live attenuated alone strategy. Additionally, these studies will allow a comparison to other DNA-based strategies currently being utilized at the Yerkes Center.